The disposal of sewage sludge is a problem. Current methods of disposing of sewage sludge include incineration, direct land or ocean application, heating and drying the sludge for sterilization and then applying it to the land, depositing it in a landfill, or granulating it with a standard rotary granulator, with heating and drying being provided by exogenous heat sources (e.g. by burning purchased fuel). While some of these methods result in what is termed a "fertilizer", such fertilizer is of relatively low analysis with regard to its "plant nutrient value".
Methods of expressing a fertilizer's "plant nutrient value" involve identifying the fertilizer's "NPK" value, wherein N relates to the amount of nitrogen, P relates to the amount of phosphorus (expressed as P.sub.2 O.sub.5), and K relates to the amount of potassium (expressed as K.sub.2 O). Thus, as reported by Wilson in U.S. Pat. No. 3,050,383 (Aug. 21, 1962), sewage sludge with a 2.5-2.5-0 value contains two and a half percent nitrogen, two and a half percent phosphorous as P.sub.2 O.sub.5, and zero percent potassium as K.sub.2 O. Except as otherwise indicated by usage, all percentage values used herein are weight-based percentages (i.e. w/w).
Fortunately, methods exist for enhancing the nutrient value of relatively low analysis organic waste material. For instance, in the aforementioned Wilson patent (the contents of the entirety of which are incorporated by this reference), a method is disclosed for treating dried animal manure and sewage sludge with controlled amounts of an acid, such as sulfuric acid, phosphoric acid (or an equivalent phosphorous compound, the strength of which is expressed as phosphoric acid), or mixtures thereof, and an aqueous ammoniacal solution, such as aqueous ammonia or ammoniacal nitrogen salt-containing solutions and tumbling the resulting reaction mass to form fertilizer granules having an "upgraded" or "enhanced" plant nutrient value.
Other methods of enhancing the plant nutrient value of relatively low analysis organic waste material with acids, bases, or mixtures thereof have also been described. See, e.g., U.S. Pat. No. 4,743,287 (May 10, 1988) to Robinson, U.S. Defensive Publication T955,002 (Feb. 1, 1977) to Norton et al., U.S. Pat. No. 5,466,273 (Nov. 14, 1995) to Connell, U.S. Pat. No. 5,125,951 (Jun. 30, 1992) to Lahoda et al., U.S. Pat. No. 5,118,337 (Jun. 2, 1992) to Bleeker, U.S. Pat. No. 5,393,317 (Feb. 28, 1995) to Robinson, and U.S. Pat. No. 5,422,015 (Jun. 6, 1995) to Angell et al.
A further drawback of sludges treated in conventional manners (e.g., by drying and screening) is that they are usually of insufficient size and shape to be spread by commonly used agricultural fertilizer spreaders, and cannot be used in the newer pneumatic spreaders.
It would be an improvement in the art if a relatively simple process existed for processing relatively low analysis organic waste material to an enhanced plant nutrient value composition, especially if such a process yielded a product which was sized and shaped to be spread by presently commercially available spreaders.